Polyamideimide (PAI) polymers are used for many high performance coating applications due to their excellent temperature resistance and high strength. The primary route to synthesizing polyamideimide polymers in a form that is convenient for the manufacture of coatings is by reacting diisocyanate, often 4,4′-methylene diphenyldiisocyanate (MDI) with trimellitic anhydride (TMA). In this process, PAI polymers are typically synthesized in polar aprotic solvents such as N-methyl amide compounds including dimethylformamide, dimethylacetamide, N-methylpyrrolidone (NMP), N-ethylpyrrolidone. See for example U.S. Pat. Nos. 2,421,021; 3,260,691; 3,471,444; 3,518,230; 3,817,926; and 3,847,878. The typical polymer solids level achieved in this synthetic route is 35-45% which may be diluted further with diluents depending on the end-use coating application.
Alternate solvents such as tetrahydrofuran, methyl ethyl ketone, gamma-butyrolactone, or dimethyl sulfoxide have drawbacks such as too low a boiling point for use as reaction solvent, low polymer solubility, or poor storage stability, which may change the application performance of the polymer resin.
U.S. Pat. Nos. 4,950,700 and 5,095,070 recite examples of gamma-butyrolactone with N-methylamide co-solvents and dimethylol ethylene urea as replacement solvents to synthesize PAI resin. However gamma-butyrolactone has neurological properties that make it subject to regulation and unsuitable for general use in formulations. Dimethylol ethylene urea has not had extensive toxicological studies performed and contains an N-methylamide functionality suspected of negative environmental and health impacts. New solvents, such as those described in U.S. Patent Application Publication No. 20100076223A1, for example 3-methoxy-N, N-dimethylpropionamide, may be too expensive or have not been fully tested for long term toxicity.
In addition, protic solvents such as ethyl lactate and propylene glycol are not suitable for use as PAI reaction media.
On a practical level, these solvents known in the art, while they may be useful for manufacturing PAI or effective for other organic synthetic reactions, are also recognized for having toxicity concerns. Thus it is an advantage to produce polyamideimide polymers using a synthetic method with minimal health and safety impact.